A spring clutch mechanism includes an internal drive member, an external driven member, and a coil spring acting on the drive member and the driven member. The drive member extends through an interior of the driven member. The coil spring has one end portion fixed to the driven member, and another end portion spirally wound over the drive member. The rotation of the drive member causes decrease in inner diameter of the coil spring to drivingly connect the drive member and the driven member through the coil spring. Such arrangement is described in Japanese Patent Application Publication No. H08-74889.
According to the disclosed spring clutch mechanism, co-axial arrangement between the drive member and the driven member may not be maintained particularly in case of high speed rotation of the drive member, thereby making impossible to realize the high speed rotation. Further, the coil spring may be disengaged from the drive member due to vibration imparted on a power tool such as a screw driver.